The present invention relates generally to digital-to-analog converters, and more particularly to an oversampling digital-to-analog converter using a delta-sigma demodulator.
As described in a paper titled "A CMOS Stereo 16-bit D/A Converter for Digital Audio", Peter J. A. Naus et al, IEEE Journal of Solid-state Circuits, Vol. SC-22, No. 3, June 1987, pages 380-395, known oversampling digital-to-analog converter includes an oversampling section in which for sampling 16-bit PCM 44.1-kHz (f.sub.S) parallel input data is sampled at 11.2896 MHz so that the sample rate is increased to 256f.sub.S. The word length is reduced to 1 bit by a noise-shaping code conversion section (or delta-sigma modulator) and the 1-bit code passes through a 1-bit D/A converter. A small analog post-filter completes the D/A conversion of the original 16-bit PCM signal. The 1-bit D/A converter comprises a switched-capacitor circuit which modulates a reference voltage with the output of the noise-shaper, an operational amplifier for amplifying the modulated reference DC voltage through a leaky integrator that feeds back the amplified signal to the input of the amplifier. Although a distortion of 90 dB was obtained, the prior art D/A conversion technique fails to attain the level of 16-bit resolution since this level of resolution requires twice as much high sampling rate. However, the increase in sampling rate represents an increase in power consumption and difficulties in circuit design.
It is also known that the performance of an oversampling D/A converter can be improved by increasing the number of discrete threshold levels of the comparator of the delta signal modulator, instead 1 of increasing the sampling rate. For example, two reference voltage sources would be required for generating positive and negative voltages of the same value if the comparator is designed to produce a three-valued output. However, if the absolute values of the reference voltages are not equal to each other, the difference between the reference voltages would represent a signal distortion.